1. Field of the Invention
The present invention relates to a magnetic tape cassette for used in an apparatus having a light source and a pair of light sensors to detect a tape end of the magnetic tape cassette in which a cassette housing is combined with transparent or semi-transparent upper half and lower half.
2. Description of the Related Art
The assignee of the present invention disclosed a magnetic tape cassette (hereinafter referred to as xe2x80x9ctape cassettexe2x80x9d) for preventing an erroneous detection of a tape end irrelevant to material of a cassette housing and for being capable of mass production, and for having a color of higher transparency which color the manufacturer intends for such the tape cassette. The tape cassette in accordance with the related art is described as follows with reference to FIGS. 9 through 12.
FIG. 9 is a plan view of a lower half of a tape cassette for having a light-shielding member for preventing an erroneous detection of the tape end caused by receiving too much light at a light sensor incorporated in an apparatus such as VCR.
In FIG. 9, a tape cassette 1 is composed of a lower half 1a and an upper half 1b (not shown), and includes a room 20 for containing a take-up (TU) reel 23 on which a magnetic tape is winded up, another room 21 for containing a supply (SP) reel 24 on which a magnetic tape is supplied to the TU reel 23, a light hole 3 for inserting a light source incorporated in an apparatus not shown, a pair of light sensors 5a and 5b incorporated in the apparatus which receive light emitted from the light source through a pair of light paths 4a and 4b respectively, a light exits 701 and 702 for passing the light emitted from the light source, ribs 801 and 802 formed together with the lower half 1a, a light shielding member 2a and 2b for limiting an irregular reflection caused by the light emitted from the light source from coming into the light sources 5a and 5b. The portion A and B in FIG. 9 show the position where the light shielding members 2a and 2b are placed in the tape cassette 1.
In FIG. 9, the light emitted from the light source inserted in the light hole 3 reaches the light sensors 5a and 5b through the light paths 4a and 4b respectively. On the other hand, the both ends of the magnetic tape (not shown) are connected to the SP reel 24 and TU reel 23 respectively through an intermediary leader tape (not shown). The magnetic tape forms a tape travel path (not shown) to connect to the SP reel 24 and the TU reel 23, wherein the tape travel path crosses the light paths 4a and 4b. 
Generally, in the tape cassette made by low light transmittance material (such as black colored material), the light emitted from the light source reaches to the light sensor 5a and 5b incorporated in the VCR through the light paths 4a and 4b respectively. As the leader tape is not having a magnetic powder, the light emitted from the light source can reach to the light sensors 5a and 5b through the leader tape. To the contrary, the magnetic tape blocks the light emitted from the light source. As a result, the tape end can be detected by the difference of amount of light received at the light sensors 5a and 5b when the leader tape passes the light emitted from the light source to the light sensors 5a and 5b. 
FIG. 10 is a cross-sectional view of the tape cassette 1 having an upper half 1b and a lower half 1a with a thickness T, made of a transparent or a semi-transparent material without having the light shielding member 2a between the light hole 3 and the light sensor 5a, wherein the light source LS is inserted in the light hole 3.
In FIG. 10, the light emitted from the light source LS will be reflected irregularly inside the tape cassette 1 as an irregular light 6. As the light shielding member 2a is not placed between the room 20 for the TU reel 23 and the light sensor 5a, the irregular light 6 will also reach to the light sensor 5a. 
In case that the tape cassette 1 consists of an upper half 1b and a lower half 1a made of a transparent or semi-transparent material, the light sensors 5a and 5b may receive the irregular light 6 even when the magnetic tape crosses the light paths 4a and 4b that the apparatus may erroneously determine that the tape end has been detected.
Accordingly, the assignee of the present invention discloses a tape cassette shown in FIG. 11 having a light shielding member 2a placed at the rib 801 of the tape cassette 1.
FIG. 11 is a cross-sectional view of the tape cassette 1 shown in FIG. 10 for having the light shielding member 2a between the light hole 3 and the light sensor 5a. As shown in FIG. 11, the irregular light 6 is blocked by the light shielding member 2a and will not reach to the light sensor 5a. 
FIGS. 10 and 11 show the relation between the room 20 for the TU reel 23 and the light sensor 5a, but of course, the relation of the room 20 and the light sensor 5a can be applied to the relation of the other room 21 for the SP reel 24 and the light sensor 5b as well.
FIG. 12 is a perspective view of the portion A shown in FIG. 9 for explaining the light shielding member 2a. In FIG. 12, the light shielding member 2a has a light window 11 to pass only the light emitted from the light source through the light path 4a, and blocks the irregular light 6. The light shielding member 2a is fixed at the rib 801 by an adhesive.
As described above, the irregular light 6 reflected diffusely inside the upper half 1b and the lower half 1a can be blocked by placing the light shielding member 2a and 2b on the light paths 4a and 4b at the ribs 801 and 802 of the tape cassette an a be prevented from reaching to the light sensors 5a and 5b. 
However, as the upper half 1b and the lower half 1a are being made of a transparent or semi-transparent material, the light emitted from the light source LS may pass through the material itself as an irregular light 6b and radiated from the edge of the upper and lower half (the edge 100), and eventually reaches the light sensor 5a. This irregular light 6b is difficult to be blocked by only the light shielding member 2a. 
Such the irregular light 6b radiated from the edge 100 may reach to the light sensor 5a and the VCR may erroneously detect the tape end. In order to prevent the VCR from this malfunction to erroneously detecting the tape end, the irregular light 6b should be blocked. This may happen to the portion B in FIG. 9 that such the irregular light 6b may reach to the light sensor 5b, which also should be blocked.
Accordingly, in consideration of the above-mentioned problems of the related art, an object of the present invention is to provide a magnetic tape cassette used in an apparatus having a light source (LS) and a light sensor (5a, 5b) for detecting a tape end of a magnetic tape being loaded therein, the magnetic tape cassette including, a cassette housing being formed by combining an upper half (1b) and a lower half (1a) made of transparent or semi-transparent member, wherein the cassette housing is equipped with a reel being connected to the magnetic tape through an intermediary leader tape having higher light transmittance for winding up the magnetic tape, a light exit (7A, 7A1, 7A2) being opposed to the light sensor (5a, 5b) when the magnetic tape cassette is loaded to a predetermined position in the apparatus, for passing light from the light source (LS) to the light sensor (5a, 5b) through the light exit (7A, 7A1, 7A2), wherein a light path (4a, 4b) of the light emitted from the light source (LS) crosses a travel path of the magnetic tape, and a rib (8) being formed in the upper half (1b) and the lower half (1a) wherein a plurality of triangular shaped prism (8a) is formed serially on at least one side surface of the rib contiguous to the light exit (7A, 7A1, 7A2).
Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.